Method and arrangement relating to multicarrier power amplifiers

ABSTRACT

The present invention relates to a Multi Carrier Power Amplifier, MPCA, comprising a number of parallel Multi Carrier Power Amplifiers, each comprising an amplification loop and an error cancellation loop, said amplification loop comprising at least a first gain and phase control network and a first amplifier and a second amplifier. The MPCA comprises an external loop comprising a second Spread Spectrum Signal added to said each first loop.

This application is the U.S. national phase of international applicationPCT/SE02/01111, filed Jun. 7, 2002, which designated the U.S., theentire contents of which are hereby incorporated by reference.

1. Technical Field of the Invention

Generally, the present invention relates to a method and arrangement forenhanced linearity of a Multi Carrier Power Amplifier (MCPA).

2. Related Art and Other Considerations

To provide flexible communications systems, it is important to mixdifferent modulation schemes, alter the number of carriers, and changerapidly between carrier frequencies without having to mechanicallyretune filters, reconfigure lossy combining networks, or add additionalpower amplifier stages for each new transceiver. A linear MCPA is asolution that provides the flexibility needed by handling a large numberof signals with the same or mixed modulation formats.

In a RF amplifier for several simultaneous signals; the amplifier mustbe linear to suppress undesired frequency components. A known techniquefor linearizing an amplifier is feedforward, according to which theamplifier is linearized by adding an error signal in anti-phase.

FIG. 1 is a block diagram illustrating a known MCPA with feedforwardlinearisation. The circuit 10 comprises gain and phase control networks11 and 12, amplifiers 13, 14, delay elements 15, 16, controller 17 andspread spectrum generator(s) 18. Inputs to the first amplifier 13 ormain amplifier are from the first gain and phase control network 11 andspread spectrum generator 18. The output from the amplifier 13, whichgenerates the output power and intermodulation products, is delayed bymeans of a delay element 15. The second amplifier 14 or error amplifierhas as input the output of the second gain and phase control network 12,which is connected to the output of the first amplifier 13 and the delayelement 16. The gain and phase control network 12 is controlled throughthe controller 17, which based on the (feedforward) input signal, spreadspectrum signal and the fed back feedforward output from the output ofthe amplifier controls the gain and phase control network.

To be able to control the cancellation of the undesired frequencies(i.e. inter-modulation products) in point (E), the spread spectrumsignal is injected in Loop 1 and detected in Loop 3. The detected spreadspectrum signal from Loop 3 is used to minimize the spread spectrum in(E).

The linearity for a feedforward amplifier depends on the linearity ofthe main amplifier and the quality of the cancellation in (E). Thecancellation bandwidth depends on the frequency response quality takinginto account the amplitude, phase and delay correspondence in bothbranches.

The amplitude and phase deviation over the frequency in the errorbranch, i.e. loop 2, compared to the main branch results in that optimalintermodulation (IM) suppression is only achieved within a limitedfrequency band around the centre frequency of the spread spectrumsignal.

The inputs to the MCPA are signals f₁ and f₂, which are connected (A) tothe gain and phase control networks 11 and a small amount is connectedvia the reference branch (B) to the Loop 1 cancellation point (C).

A small part of the output power from amplifier 13 is coupled to theLoop 1 cancellation point (C). The delay 16 (time delay) is arranged toproduce a delay equal to the delay in the main branch. In (C) the inputsignal f1, f2 is added to the amplified signal, with the same amplitudebut in anti-phase, i.e. f1 and f2 will be cancelled and only the IMproducts from the main amplifier 13 are left.

In Loop 2, the input signals in the error branch are connected to thegain and phase control network 12 and then to the error amplifier 14.The time delay 15 of Loop 2 provides a time delay, which is equal to thetime delay of the error branch. Loop 2 is adjusted so that IM productsfrom the main amplifier 13 are cancelled in point (E).

Loop 3 controls the amplitude and phase of Loop 2 in the error branch.

A Spread Spectrum Signal (SSS) is injected before the main amplifier 13,which is amplified together with the original signal. In (F) some of thetotal power, containing f1 and f2, IM products and SSS, is decoupled. Byeliminating f1 and f2 and comparing the signal with the same SSS asinjected in Loop 1, the-amplitude and phase can be controlled in Loop 2of the error branch. Thus, both SSS and IM products are cancelled in (E)

A super-linear feedforward amplifier, for amplifying radio-frequencyinput signals produced in one or more frequency channels over an inputband is known through U.S. Pat. No. 6,166,601, which includes aradio-frequency power amplifier, which amplifies the signals. A signalcancellation circuit loop generates an error signal responsive todistortion products in the amplified signals. A digital correction blockdigitally equalizes the input signals responsive to a transfer functionof the amplifier, whereby the input signals are substantially cancelledout of the error signal over the entire input band. An errorcancellation circuit loop subtracts the error signal from the amplifiedsignals to generate a linearized output signal.

In U.S. Pat. No. 6,606,984, an amplifier is described, which includes anamplifying section for simultaneously amplifying a plurality of inputsignals having different frequency bands, a distortion extractingsection for separating and extracting at every different frequency bandsa distortion component from a plurality of signals having the differentfrequency bands outputted from the amplifying section, a distortioncompensation section for adjusting independently at every differentfrequency band the phase and the amplitude of the distortion componentseparated and extracted at every different frequency band and adistortion eliminating section for cancelling and outputting adistortion component from a plurality of signals having the differentfrequency bands outputted from the amplifying section based on thedistortion component adjusted at every different frequency bands. Theabove distortion compensation section adjusts independently at everydifferent frequency band the phase and the amplitude of the distortioncomponent separated and extracted at every different frequency bands insuch a manner that the above-mentioned phase and amplitude agree withthe phases and the amplitudes of the distortion components contained ina plurality of signals having the different frequency bands outputtedfrom the amplifying section.

WO 0003479 relates to a multi-carrier power amplifying arrangement,which comprises a number of amplifying means each of which includes amain amplifier and linearizing means. Furthermore it comprises signaladding means for adding the adding output signals from the amplifyingmeans in phase, error detecting means for detecting the average phaseerror of the amplified signals and for providing a compensating controlsignal. Said compensating control signal is provided to each amplifyingmeans at least to compensate for the average phase error.

U.S. Pat. No. 5,770,971, relates to a control arrangement and method forcontrolling a gain and phase adjuster used for controlling the gain andphase of a compensation signal for compensating for the distortionproduced by a power amplifier. The control arrangement makes use of areference signal, of known frequency, which is amplified along with thedesired carrier signals. The reference signal component of the amplifiedsignal is then isolated and a comparison is made, either with the actualreference signal, e.g., by a QAM demodulator, or alternatively with theknown frequency of the reference signal, e.g., by a FM discriminator, inorder to determine the differences in gain and in phase of the referencesignal component of the amplified signal in comparison with thereference signal. For example, a QAM demodulator is used for producing afirst difference signal indicative of the phase switch difference, and asecond difference signal indicative of the gain switch difference. Thesesignals are then correlated with the signal envelope of the main signalin order to produce signals indicative of the sign and magnitude of thecontrol signals used to adjust the gain and phase adjuster in a feedback loop.

A linear power amplifier and method for removing the intermodulationdistortion with a predistortion system and a feed forward system isdisclosed in U.S. Pat. No. 5,877,653. The linear power amplifier havinga main power amplifier includes: a predistortion system for firstlysuppressing the intermodulation signal generated upon amplification ofan RF signal in the main power amplifier, by generating a harmonicscorresponding to the input RF signal and a predistortion signal withcoupling the RF signal to the harmonics; and a feed forward system forsecondly suppressing the intermodulation signal by cancelling the inputRF signal and the output of the main power amplifier, extracting anintermodulation signal distortion, error-amplifying the extractedintermodulation signal distortion, and coupling the amplifiedintermodulation signal with the output of the main power amplifier.

BRIEF SUMMARY

An MCPA comprises a number of parallel Multicarrier Power Amplifiers,each comprising an amplification loop and an error cancellation loop.The amplification loop comprises at least a first gain and phase controlnetwork and a first amplifier. The error cancellation loop comprises atleast a second gain and phase control network and a second amplifier.

In addition, for enhanced linearity, the MCPA includes an additionalexternal loop and also an adaptive optimisation of the amplitude andphase rate substantially over the desired frequency band.

For this reason the initially mentioned MCPA comprises an external loopcomprising a second Spread Spectrum Signal added to said each firstloop. The input signal to each parallel MCPA stage and a directionalcoupler at outputs of said parallel MCPAs is arranged to couple theoutput signal to a control arrangement to detect a total residualintermodulation signal and the second Spread Spectrum Signal. The gainand phase control networks are controlled by means of said controlarrangement.

Preferably, all MCPAs include an additional vector modulator in form ofa gain and phase control network.

In one embodiment, an equalizer arrangement is provided in saidcancellation loop of each MCPA. Preferably, but not exclusively, theequalizer arrangement is provided between said gain and phase controlnetwork and said second amplifier. The equalizer can be controlled indifferent sub-bands by controlling a centre frequency and/or bandwidthof said spread spectrum signal in different sections of the frequencyband and detecting the variations. The equalizer can be controlled byemploying several different spread spectrum signals having differentcodings.

The invention also relates to a method of enhancing intermodulationsuppression in a Multicarrier Power Amplifier, MCPA, comprising a numberof parallel Multicarrier Power Amplifiers, each comprising anamplification loop and an error cancellation loop. The amplificationloop comprises at least a first gain and phase control network and afirst amplifier. The error cancellation loop comprises at least a secondgain and phase control network and a second amplifier. The methodcomprises providing said MCPA with an external loop comprising a secondSpread Spectrum Signal added to said each first loop.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be further described in anon-limiting way with reference to the accompanying drawings in which:

FIG. 1 is a block diagram illustrating an MCPA according to prior art,

FIG. 2 illustrates a block diagram of a first example embodiment of aMCPA,

FIG. 3 illustrates a block diagram of a second example embodiment of aMCPA, and

FIG. 4 illustrates a block diagram of a third example embodiment of aMCPA.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As described herein, the intermodulation suppression product for anumber of parallel MCPAs is improved by providing an external loop. Theoptimal intermodulation (IM) from each parallel MCPA is expected to havedifferent amplitude and phase. The total IM signal is thus smaller, i.e.a certain cancellation is achieved.

An example according to the first aspect of the technology isillustrated in FIG. 2. In the figure, the function of each MCPA 20′ and20″ is the same as described above and parts having the same referencesigns have the same function.

In FIG. 2, an additional Spread Spectrum Signal (SSS) 210 is added inall the first loops together with the first Spread Spectrum Signal 28.The Spread Spectrum Signal (SSS) 210 is also connected to thecontrollers 27′, 27″ in each Loop 2. Using a directional coupler 230 atall outputs of the MCPAs, the total residual IM signal can be detected.The gain and phase control network is controlled by means of a centralcontroller 220. All MCPAs can be provided with gain and phase controlnetwork as additional vector modulators. In this way the total IM signalcan be cancelled.

The MCPAs 20′ and 20″ comprise gain and phase control networks 21′, 21″and 22′, 22″, amplifiers 23′, 23″, 24′ and 24″, delay elements 25′, 25″,26′ and 26″, controller 27′, 27″ and spread spectrum generator 28′ and28″. Inputs to the first amplifiers 23′, 23″ or main amplifiers are fromthe first gain and phase control networks 21′, 21″ and spread spectrumgenerator 28″. The output from the amplifier 23′, 23″ is delayed bymeans of delay element 25′, 25″. The second amplifiers 24′, 24″ or erroramplifiers have as input the output of the second gain and phase controlnetwork 22′, 22″, which is connected to the output of the first gain andphase control network 21′, 21″ and the delay element 26′, 26″. The gainand phase control networks 22′, 22″ are controlled through thecontroller 220, which uses the (feedforward) input signal, spreadspectrum signal and fed back feedforward output from the output of theamplifier.

In a more preferred embodiment, as illustrated in FIG. 3, according thesecond aspect of the technology, the control and separate spreadspectrum signals to each MCPA are substituted by one spread spectrumsignal 310 and one controller 320.

Accordingly, the Spread Spectrum Signal 310 is added in all the firstloops. Using a directional coupler 330 at all outputs of the MCPAs, thetotal residual IM signal can be detected. The gain and phase controlnetwork is controlled by means of the central control unit 320. In thisway the total IM signal can be cancelled.

In this way, the Loop 3 of each MCPA is substituted by one central Loop3. According to this embodiment the number of control circuits isreduced by N−1, wherein N is the number of MCPAs. This allows moresimple MCPAs having the same performance at higher output power.

In yet another embodiment, as illustrated in FIG. 4, which correspondsto the embodiment of FIG. 2, equalisers 29′ and 29″ or a circuits havingsimilar functionality are introduced into Loop 2 of the MCPAs 20′ and20″.

Through introduction of the equalizer in Loop 2, i.e. between the outputof the gain and phase control network 22′, 22″ and the input of theerror amplifier 24′, 24″, which adjust the amplification in differentparts of the frequency band, the suppression in loop 2 can be optimisedover the desired frequency band.

Thus, the amplitude and/or phase variations in all parts, i.e. the gainand phase control networks 22′, 22″, of the error branch, i.e. gain andphase control networks 22′, 22″ and the amplifier 24′, 24″, can becorrected. The differences over the frequency band are detected in Loop3, with respect to which the equalizer 29′, 29″ is controlled tominimize the differences. Thus, variations over time period anddepending on the temperature, operation power, and different operationcases can be compensated for, which allows improvement of theintermodulation suppression.

In one preferred embodiment, it is also possible to control the centrefrequency and/or bandwidth of the spread spectrum signal in differentsections of the frequency band and detect the variations in loop 3.Thereby, it is possible to control the equalizers in different bandparts.

It is also possible to control the equalizer by employing several spreadspectrum signals having different codings. In this case a separatedetection in loop 3 is needed. Moreover, the equalizer can be arrangedin any part of Loop 2.

It is also possible to introduce equalisers in loops 2 of the embodimentaccording to FIG. 3, achieving same benefits as in the embodiment ofFIG. 4.

Additionally but not exclusively, the MCPA according to the presentinvention can be used in base station amplifiers, which providesenhanced efficiency, power, bandwidth and IM suppression.

The invention is not limited to the shown embodiments but can be variedin a number of ways without departing from the scope of the appendedclaims and the arrangement and the method can be implemented in variousways depending on application, functional units, needs and requirementsetc.

1. A Multicarrier Power Amplifier (MCPA) comprising: plural parallelMulticarrier Power Amplifiers, each Multicarrier Power Amplifiercomprising an amplification loop and an error cancellation loop, saidamplification loop comprising at least a first gain and phase controlnetwork and a first amplifier, said error cancellation loop comprisingat least a second gain and phase control network and a second amplifier;an external loop comprising an external loop Spread Spectrum Signaladded to each first loop; wherein the gain and phase control networksare controlled by a control arrangement, the control arrangementreceiving as input signals a feedforward input signal, a fed back feedforward output signal, and the external loop Spread Spectrum Signal. 2.The MCPA according to claim 1, further comprising a directional couplerconnected to outputs of the plural parallel MCPAs, and wherein an inputsignal to each parallel MCPA and the directional coupler are arranged tocouple the output signal to the control arrangement for detecting atotal residual intermodulation signal and the external loop SpreadSpectrum Signal.
 3. The MCPA according to claim 1, wherein the MCPAscomprise an additional vector modulator comprising a gain and phasecontrol network.
 4. The MCPA according to claim 1, further comprising anequalizer arrangement provided in the cancellation loop of each MCPA. 5.The MCPA according to claim 4, wherein the equalizer arrangement isprovided between said second gain and phase control network and saidsecond amplifier.
 6. The MCPA according to claim 4, wherein theequalizer is controlled in different sub-bands by controlling a centrefrequency and/or bandwidth of the exterior loop spread spectrum signalin different sections of the frequency band and detecting thevariations.
 7. The MCPA according to claim 6, wherein said equalizer iscontrolled by employing different spread spectrum signals havingdifferent codings.
 8. A Multicarrier Power Amplifier (MCPA) comprising:plural parallel Multicarrier Power Amplifiers, each Multicarrier PowerAmplifier comprising an amplification loop and an error cancellationloop, said amplification loop comprising at least a first gain and phasecontrol network and a first amplifier, said error cancellation loopcomprising at least a second gain and phase control network and a secondamplifier; a directional coupler connected to receive and use outputsfrom each of the plural Multicarrier Power Amplifiers for generating afeedforward output signal; an external loop wherein an external loopSpread Spectrum Signal is added to each first loop; a controlarrangement included in the external loop for controlling the gain andphase control networks of the plural Multicarrier Power Amplifiers, thecontrol arrangement receiving as input signals a feedforward inputsignal, the feed forward output signal generated by the directionalcoupler, and the external loop Spread Spectrum Signal.
 9. A method ofenhancing intermodulation suppression in a Multicarrier Power Amplifier(MCPA), the MCPA comprising plural parallel Multicarrier PowerAmplifiers, each Multicarrier Power Amplifier comprising anamplification loop and an error cancellation loop, said amplificationloop comprising at least a first gain and phase control network and afirst amplifier, said error cancellation loop comprising at least asecond gain and phase control network and a second amplifier; the methodcomprising: providing said MCPA with an external loop comprising anexternal Spread Spectrum Signal added to each first loop and with acontrol arrangement controlling the gain and phase control networks; andapplying a feed forward input signal, a fed back feed forward outputsignal, and a second Spread Spectrum Signal to the control arrangementas input signals.